The present invention relates to a piston ring for use with a piston in an internal combustion engine, and more particularly relates to a surface structure for the cylindrical outer surface of the ring which slides on the cylinder or the cylinder liner of the engine and accordingly requires high wear and seizure resistance.
Thermal load has increased in modern internal combustion engines, due to their high rotational speed and their high compression ratios, which are employed in order to produce high performance and in connection with exhaust emission control. Thereby, more load is imposed on the mutually sliding areas of piston rings and cylinders. Especially, in diesel engines, the generation of carbon soot has increased the requirement for high seizure resistance, wear resistance, and scuffing resistance.
Conventional piston rings are known whose sliding surfaces are coated with chromium or molybdenum. Cr coated piston rings have good wear resistance, but they are prone to seize or scuff when used with cylinder liners made of cast iron. Mo coated piston rings have good seizure resistance, but when used in engines which subject them to high thermal load there is a tendency for oxidized layers to be formed in the boundary region between the piston ring mother metal (usually cast iron) and the deposited Mo layer, and also between the deposited metal particles, resulting in poor adherence of the deposited layer to the mother metal. Furthermore, Mo is expensive.
It has been proposed and practiced to produce piston rings which have several layers of coating on their outer cylindrical surfaces. For example, layers of Mo alloy and Fe alloy; layers of Mo alloy and ceramics; and layers of Fe alloy and ceramics. However, each of these combinations has both advantages and disadvantages, and such piston rings are not used very much at present in actual engines.